The present invention is particularly adaptable for use in electronic organs. The term "organ" is used throughout the specification to include any electronic musical instrument having a keyboard, such as electronic organs, electric pianos, and accordions. The principles of the present invention are, in fact, applicable to any musical instrument in which musical sounds are generated in response to the actuation of key switches regardless of whether those switches are actuated directly, i.e., by the musician's fingers, or indirectly, e.g., by the plucking of strings. The term "key" includes depressible levers, actuable on-off switches, touch- or proximity-responsive (e.g, capacitance- or inductance-operated) devices, closeable apertures (e.g., a hole in a "keyboard" of holes which, when covered, closes or opens a fluidic circuit to produce a tonal response), and so forth.
The function of an electronic organ is to faithfully reproduce, or simulate by electronic means, the sounds or tones developed by a true pipe organ in response to playing of the organ by the organist a selection of notes, and voices and other characteristics of those notes. Generally, true pipe organs are out of reach of the public because of their substantial expense and size. Thus, electronic organs have been developed as a substitute which is more readily available.
Electronic musical instruments usually contain a limited number of audio channels through which tonal information can be processed. Each channel usually handles multiple waveforms for reasons of economy, since providing a dedicated audio channel for each tone can be very costly.
It is well known in the art of electronic tone generation that generating two or more simultaneous notes in a single audio channel, where the tones have pitches that are the same or very close to the same but whose phases are not synchronized, will result in tone color distortion. This distortion is the result of random phase relationships associated with the combined signals, where some harmonics of the signals may add while others cancel. Specifically, generating more than one non-phase synchronized tone could result in a different sound being created on successive key depressions. Additionally, listening to simultaneously produced non-phase synchronized tones may not sound as one would expect after hearing each sound individually.
An important element of any tone generation system is the tone generator. The development of tone generation systems has evolved in a similar manner to almost all other electronic systems. Specifically, at one point in time electronic instruments were designed and built using analog systems. At that time, tone generators were necessarily built using analog designs. Eventually, however, digital systems were incorporated into electronic musical instruments and therein digital tone generators became commonplace. U.S. Pat. No. 3,610,799 discloses the basic design for digital tone generators. The tone generator uses an accumulator output as a memory address pointer for a memory containing sample waveform information. The information retrieved from the memory is applied to an envelope generator wherein the waveform information is increased or decreased in value to create a pleasant attack and decay.
The frequency of the resultant audio signal from the tone generator is a function of an increment number by which the memory containing the waveform information is indexed. In other words, the increment number determines the amount by which the memory address will be incremented to point to the next desired waveform sample. The larger the increment number used for addressing the waveform data, the higher the resulting audio frequency will be. The multibit accumulator which provides the memory address accepts as an input the increment number which is a numerical value proportional to the desired audio frequency. This numerical value, termed a "frequency number," is related to the depressed key on the musical instrument. In operation, the accumulator outputs a larger number each time it adds the frequency number to the previous sum. This value is used as a memory address for the memory containing the waveform information. The larger the frequency number, the more rapid the accumulation will be and thus the higher the audio frequency will become. Each of the keys on a keyboard has an associated frequency number consistent with the required audio frequency to be produced upon depression of that particular key.
As is well known, in electronic organs as well as pipe organs there are included a plurality of devices called stops. Some of these stops serve to activate various ranks. In pipe organs, a rank is a set of pipes which produce sounds representative of a particular instrument (e.g., flute) or a particular general musical sound for a particular keyboard. When the stop is activated, a particular rank associated with that stop is activated to sound when a key on that keyboard is depressed. A stop may also serve to modify functional characteristics of the keyboard. For example, the stop may require that when a particular key is depressed a sound for a key an octave higher is produced in addition to the sound for to the depressed key. Additionally, a stop could link distinct keyboards of the organ. These types of stops are known as modifier stops. These modifier stops affect the ranks by, for example, linking certain ranks or indicating that the rank should sound when otherwise it would not. In the electronic organ, the rank maintains information about waveform information representative of the sounds a corresponding pipe organ rank would make.
In electronic organs, in order to track the notes for which one desires to generate a sound, the note is described by a combination of parameters. These parameters must indicate at least the particular key which has been struck, on which keyboard the particular key resides, and whether the keyboard has been modified by any stops. One particular scheme for tracking the notes to be generated is to use a key number and a keying group. The key number is a number associated with a particular key position on any of the physical keyboards. A keying group numbers is a number which differentiates notes coming from different physical keyboards or from a key which has been associated with a keyboard in response to a modifier stop being activated.